I'm away from my computer right now, but I think once you've registered "b" you should be able to register "a.b" with whatever settings you like, including setting it as a delegate.

What if I want to make a child account a delegate? How do I go about doing that?

That was helpful, thanks!

The period character designates child accounts.  So to register "a.b" you have to be the owner of the "b" account already.

This allows you to have multiple accounts for special purposes that are all publicly attached to the same identity.

I've been trying to register a delegate account on the blockchain. It keeps giving me an error message, saying that the account hasn't been approved by the adult account. I've tried the console method, and that doesn't seem to be working either. Anyone have any suggestions?